The invention relates to a display device for displaying alpha-numerical characters, which device comprises an electro-optical material between a first supporting plate having at least one electrode and a second supporting plate having a plurality of electrode segments.
A display device of this type is known from U.S. Pat. No. 3,781,864. In this Patent a liquid crystal display device is shown with a counterelectrode on the one supporting plate and 7 electrode segments for each character on the other supporting plate, and it describes the drive mode for such a display device. Substantially only numerals are shown while using the so-called direct drive method. In this drive method a segment to be visualized is continuously driven, while the polarity of the drive voltage is periodically changed so as to prevent degradation of the liquid crystal. Therefore, a separate connection is required for each electrode segment.
It has been possible to reduce the number of required connections in such display devices by splitting up the undivided electrode into two or more parts and by ensuring the drive mode by way of multiplex drive (1:2 multiplexing, 1:3 or 1:4 multiplexing). Notably at a higher multiplex degree (5 or more) this leads to a more complicated drive mode as well as to a lower effective drive voltage across the picture segments to be driven. Notably when using liquid crystalline material as an electro-optical medium, such a lower effective voltage leads to a decrease of the viewing angle. The contrast also deteriorates at a multiplex degree of more than 4.
Since the shape of the segment electrodes is fixed, the variation of characters to be displayed is also limited in this case.
The latter is much less the case if the characters are composed of elements arranged in accordance with a matrix, for example, n rows and m columns defining nxm character elements. As n and m increase, the characters can be fixed more accurately. However, this requires a complicated drive circuit (1:n multiplexing), while the effective voltage may even further decrease due to the lower duty cycle (1/n) (n is generally high, for example, n.gtoreq.8).